The present invention relates to a circuit board or a flexible cable (Flexible Printed Circuit Board (FPC), Flexible Flat Cable (FFC), etc.), and more specifically, to a circuit board or a flexible cable with shielding planes, which have varied void opening patterns used to control both the impedance and the transmission time.
For high-speed logic circuits in digital computers, applications of microstrip and stripline structures are very often and extensively used to the interconnection between these circuits. Moreover, for the high-speed data transmission rate, the differential mode of transmission lines with shielding planes is a necessary and important method for low voltage (3.3 volts) to reduce transmission errors induced by the noise during the data transmission. There is a requirement of one hundred ohm impedance for differential mode of transmission lines. Recently, automatic techniques and equipment are capable of fabricating these structures with the controllable impedance and transmitted timing of signal paths. However, the asymmetric structure of microstrips has a serious disadvantage because of the weakness of the mechanical properties for flexible cables, and it also permits significant levels of extraneous electromagnetic radiation.
For the symmetric structure of striplines, not only can it enhance the mechanical features of flexibility and anti-fatigue for flat flexible cables, but also greatly reduce effects of electromagnetic radiation. However, due to the symmetric structure of striplines, the additional ground reference plane greatly reduces the impedance of signal path because of the increasing capacitance between the signal path and the additional ground reference plane. To consider the high flexibility and anti-fatigue with repeated motion for the stripline type of flat flexible cables, the distance between the differential mode of signal paths and the ground reference plane should be designed to be shorter than that between the pair of the differential mode of signal paths. It also implies that the same design method is needed for avoiding increasing the thickness of the printed circuit board to maintain the desired impedance at the same time.
Ground planes or other voltage reference planes are positioned in planes parallel to the conductor planes in which the conductors are designed in plane in a flexible cable or a printed circuit board. It aims to control the impedance of the conductors and to block the transmission of electromagnetic radiation from conductors carrying high frequency signals. A solid ground plane is generally used in printed circuit boards or flexible cables, but it is inflexible except for the thin film type.
Another disadvantage for solid ground plane is that the impedance of signal lines may be lower than desired because the large capacitance is built up in the small spacing between the signal lines and a solid ground plane or the reference plane. The more we increase the spacing between the signal lines and a solid ground plane or the reference plane to reduce the capacitance and thereby increase the impedance of the signal lines, the thicker and thus less flexible and more likely to break with repeated motion. Similarly, a printed circuit board becomes thicker and thus more heavy and more costly to build.
Reference planes having conductive elements formed in a grid have been utilized in microstrip designs to increase the impedance and to provide flexibility. However, because the grid is not continuous like a solid reference plane, it has been found to be quite difficult to control both impedance and transmitted timing of signal lines protected by a grid reference plane with only one pattern.
One of the particular difficulties is to control the impedance of flexible cables and printed circuit boards utilizing grid reference planes, especially for stripline type cables with the structure of turns. Generally, when the orientation of a signal line needs to be changed by, for example, 90 degrees or the like, the turn is not incorporated into the signal line with a single 90-degree turn. Rather, the change in orientation is generally implemented with a curve such that the orientation of the signal line varies continuously from its original orientation to the new orientation. It is likely that the signal line will have different alignment with the conduction of the upper or the lower grid with both grids at various points in the turn. Such alignment causes a significant variation in impedance at such points and causes a substantial impedance discontinuity.
The impedance of microstrip and stripline construction is determined by the signal conductor width, the separation of the conductor from the reference planes, the dielectrics that surround the conductor and, to a lesser degree, the thickness of the conductor. However, such traditional methods of determining impedance in striplines and microstrips impose too many constraints on the designer. For example, the impedance needs to be twice of 100 ohms for the differential mode transmission lines. One way of obtaining such high impedance using existing technology is to increase the separation between the signal conductor and the reference plane. However, this would require the use of a thicker flexible cable with the less flexibility and anti-fatigue, different dielectric constant materials surrounding the conductor, or expensive printed circuit boards of nonstandard thickness. Such nonstandard printed circuit boards are not only expensive to implement, but also undesirable in many applications due to their thickness.
There is a disadvantage associated with traditional microstrip construction with high-speed transmission rate in that it generates both forward and reverse crosstalk, which can seriously deteriorate signal quality. Crosstalk is the effect of coupling the signal of one channel into another. Crosstalk may arise from a number of sources, one of which is the unbalance of cable parameters, in particular, the capacitance and inductance between conductors. Therefore, this unbalance may result in a serious coupling from the signal of one conductor into another, and such unbalance is generally aggravated when a conductor is exposed to nonhomogeneous media, as is the case with traditional microstrip construction.
Solid surface conductors in traditional microstrip construction are most likely to radiate high levels of electromagnetic radiation to interfere with the functioning of surrounding electronics. Conversely, extraneous radiation may also affect the operation of surface conductors. In traditional microstrip construction with high-speed transmission rate, the surface of the conductor is free to radiate into the cavity of the system enclosing the circuit board, so it is difficult to provide adequate shielding. It implies that the structure of stripline is needed to reduce the containment of such radiation. However, desired high impedance conductors of stripline construction is very difficult to implement without drastically increasing the separation between reference planes and conductors. Such an undesired increase in thickness would cause problems for the case of thin circuit boards needed in notebook computers or other standard circuit board needed to reduce the cost.
Flexible reference planes are needed for a stripline type flexible cable to have the capability of thousands of flexures, to achieve desired impedance and transmitted timing that permit transfer of the signals without degrading the signal quality, and to provide an acceptable shielding capability.
Due to the slow wave effect, the transmitted time on the solid plane is faster than that of the shielding with void opening patterns. When the difference among the length of the transmission line is large enough to have timing effect for high-speed transmission rate, the traditional method is to add extra equivalent length for the short transmission lines at some place to compensate for this effect. But this traditional extra length compensation may induce an undesired electromagnetic radiation due to the discontinuity of the impedance.
Even though the transmission lines have the same length, sometimes, it is necessary to control the transmitted timing to avoid the significant harmonic mode of the high-speed transmission rate. The transmitted time is related to the dielectric constant of the surrounding material, the slow wave effect due to the shielding pattern, and the compatible long length of transmission line. If it equals the significant harmonic mode of the high speed of transmission rate, the harmonic mode of this signal will bounce back and forth between two ends of the transmission line to deteriorate the signal transmission.
The present invention is related to a flexible cable (Flexible Printed Circuit Board (FPC), Flexible Flat Cable (FFC), etc.) and a printed circuit board with shielding planes for transmission lines (preferably, low voltage differential transmission mode circuits). According to the present invention, both the impedance and transmission time for the transmission line in the cable can be controlled by shielding planes with varied void opening patterns. Capacitance and slow wave effects related to the combination of void opening patterns and the location configuration related to locations of void opening patterns are the major factors to consider.
Two major categories of varied void opening patterns are used. The first one is to design the shielding planes with a set of varied void opening patterns, and the second one is a combination of a set of varied void opening patterns (preferably, a fixed void opening pattern) and a set of small predetermined configuration of solid patterns.
It is also intended to design the transmission time and impedance with the consideration of the mechanical flexibility of the flexible cable or the thickness of the printed circuit board.
Varied void patterns are applied on the shielding plane to generate slow wave effect and to reduce the capacitance of the transmission lines which can control the transmission time and increase the impedance at the same time for matching the requirements.
Thin film type conductive elements such as silver paste on shielding planes are connected and designated as the only ground paths for the transmission lines. Suitable resistance type impedance is provided to reduce the undesired harmonic mode effect which bounces back and forth between two ends of the transmission line. At least one ground path (preferably one) for the DC power is added to reduce the power consumption of silver paste. During the turn of the cable, the crosstalk between the transmission lines is blocked by use of via located at suitable position with suitable spacing.
Therefore, the object of the present invention is to provide a printed circuit board or a flexible cable that comprises a first shielding plane with a predetermined shielding configuration, which consists of conductive elements with a first set of predetermined varied void opening patterns and a first predetermined location configuration. The predetermined location configuration is related to locations of varied void opening patterns. There are two predetermined location configurations: the first one is not related to curves of the signal lines, and the second one is related to curves of the signal lines. The printed circuit board or the flexible cable further comprises a second shielding plane with a second predetermined shielding configuration, which consists of conductive elements with a second set of predetermined varied void opening patterns and a second predetermined location configuration.
Another object of this invention is to provide a method for simplifying the design of the impedance and transmission time with the same first and second location configurations. The method may have the same set of varied void opening patterns and the same location configuration, but the first shielding plane is the mirror image of the second shielding plane. The identical shielding plane is used for further simplifying the design of the impedance and transmission time. A conducting element is positioned between the first and the second shielding planes to form the signal conductor.
Another aspect of the present invention is to provide a printed circuit board or a flexible cable that comprises a first shielding plane with a predetermined shielding configuration, which comprises conductive elements with the combination of a fixed void opening pattern (or a set of varied void opening patterns) and a small predetermined portion of solid pattern. The printed circuit board or the flexible cable further comprises a second shielding plane having a second predetermined configuration. A conducting element is positioned between the first and the second shielding to form the signal conductor.
Another aspect of the present invention is a method to select varied void opening patterns, which induce slow wave effects to compensate for this timing effect with the less undesired electromagnetic radiation.
Another aspect of the present invention is a method to select the preferable locations and preferable orientation for the small portion of solid shielding.
Another object of this invention is to provide a printed circuit board or a flexible cable with the microstrip structure that comprises a shielding plane with a predetermined shielding configuration, which consists of conductive elements with a set of predetermined varied void opening patterns and a predetermined location configuration.
Other object of this invention is to provide a printed circuit board or a flexible cable with the microstrip structure with a shielding plane. The shielding plane has a predetermined shielding configuration comprising conductive elements with the combination of a fixed void opening pattern (or a set of varied void opening patterns) and a small predetermined portion of solid pattern.
Other features and advantages of the present invention will become apparent from the following description which refers to the accompanying drawings.